Six episodes of primary hypothyroidism followed pregnancy in five women. Within 6 months post partum, all were clinically hypothyroid, with a diffuse goiter of two or three times normal size, altered thyroid function tests (average thyroxine [T4] = 2.6 micrograms/dl, thyroid-stimulating hormone [TSH] = 58.2 microunits/ml), and elevated antithyroid antibodies. (All had antimicrosomal antibody titers of 1:6,400 or more.) After 5 more months, all patients had a decrease in antibody titer, resolution of symptoms, and recovery of thyroid hormone levels (average T4 = 7.3 micrograms/dl), although two patients had compensated hypothyroidism (their average TSH = 9.3 microunits/ml). Four patients had lymphocytic thyroiditis diagnosed by fine-needle aspiration biopsy. Despite the apparent transience of postpartum hypothyroidism, all five patients have chronic thyroid dysfunction: three women have persistent goiters (all less than twice normal size) and two have compensated hypothyroidism; one patient had previous Graves' hyperthyroidism, and two had recurrent transient hypothyroidism (at 21 months postpartum in one patient, and 5 months after another pregnancy in the other). Three women had transient, painless hyperthyroidism (average T4 = 16.1) before becoming hypothyroid: two patients within 8 weeks post partum, and the third patient at 20 months (preceding an episode of recurrent hypothyroidism). Previous reports of postpartum hypothyroidism have come from scattered locales abroad, and it is unclear whether local environmental factors are pathogenic. Nevertheless, this syndrome may, in general, have gone unrecognized and may be a significant cause of illness in postpartum women.